<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>菜鸟教程(runoob.com)</title>
</head>

<body>

    <p>点击按钮在弹窗中使用换行。</p>
    <button onclick="myFunction()">点我</button>
    <p id="demo"></p>
    <script>
        function myFunction() {
            alert("Hello\nHow are you?");
        }
    </script>

</body>

</html>